This invention relates to methods and apparatus for monitoring the trigger circuit of an electrical power switch.
A prior art monitoring system for the trigger circuit of an electrical power switch is disclosed and described in the leaflet of the Swiss firm Brown, Boveri & Cie., CH-ES 64-22.1 D/E/F, Classification No. 071401, "Ausloesekreisueberwachungsrelais Typ VIR 1". The trigger circuit monitoring relay disclosed therein comprises a monitoring element with two windings, one of which is connected, through a limiting resistance, to the connecting terminal of a protective relay in the connecting wire to a power switch, and the other of which windings is connected, through a limiting resistance, to a main contact and an auxiliary contact of the power switch. An alarm relay is switchable on and off via switch contacts of the trigger circuit monitoring relay, which switch contacts are disposed in a circuit with an alarm device which device is supplied with a separate alarm voltage. In this prior art monitoring system, the trigger circuit monitoring relay does not respond to a short circuit in the circuit-breaking coil of the power switch, or to an abrupt increase in the resistance of the circuit-breaking coil (said increase due to, e.g. corrosion).
Another prior art device is disclosed in German OS No. 30 37 273, wherein an electronic trigger circuit monitoring system is disclosed with an optically coupled diode between three measuring signal input stages and a registration stage for instant visual and/or audio registration of a fault. this monitoring system operates by the current-monitoring principle, whereby the voltage drop is detected at two series-connected power diodes in the circuit of the power switch. The necessary measuring current (about 20 mA) for monitoring the function in the trigger circuit is produced by wiring the circuit-breaking trigger contacts, as well as the auxiliary contacts of the power switch, with glazed wirewound resistors. The registration stage responds when the measurement current in one of the measurement signal input stages falls below about 12 mA. In order to protect against excess voltage in the case of failure of a high-ohm power diode, in each input stage a zener diode is provided in parallel with the two power diodes. This zener diode is sized for a temporary current of 10 A. A disadvantage of this arrangement is the use of the power diodes, which are unreliable, and also the use of the auxiliary contacts of the power switch, and the parallel hookup of the glazed wirewound resistors.